Device for producing high frequencies by means of light frequency beating

Abstract

In the field of the production of very high frequencies, for example from 1 gigahertz to several terahertz, by beating the frequencies of two laser beams together, a device includes a resonant optical cavity having very stable dimensions receiving the beams, with for each beam, an interrogation device of the resonant cavity supplying an electrical signal representing the difference in frequency between the light frequency of the beam and a resonance frequency of the resonant cavity. The frequency of each beam is servo controlled to minimize the frequency difference observed. The laser beams are produced by a dual-frequency laser producing two beams of different frequencies and orthogonal polarizations. A polarization separator is used for separate servo control of the beams according to polarization, and a polarizer is placed behind a main output of the resonant cavity producing an electromagnetic beam mixing the two polarizations and amplitude-modulated at the beat frequency.

The invention claimed is: 
     
       1. A device for producing amplitude-modulated radiation at a very high frequency, from a beat frequency of two laser beams emitting different light frequencies, the device comprising:
 a resonant optical cavity having very stable dimensions receiving the two laser beams, said resonant cavity having at least two different resonance frequencies; 
 for each laser beam, a respective interrogation device of the resonant cavity capable of supplying an electrical signal representing the difference in frequency between the light frequency of the beam and a resonance frequency of the resonant cavity, and 
 servo control means for separately controlling the light frequency of each of the laser beams according to the frequency difference observed,
 wherein the laser beams have orthogonal polarizations, and the servo control means comprise a polarization splitter for the separate servo control of the beams according to their polarization, and a polarizer is placed at the output of the device on the path of the two superposed beams for producing a radiation beam for producing an electromagnetic beam combining the two polarizations and amplitude-modulated at the beat frequency. 
 
 
     
     
       2. The device as claimed in  claim 1 , wherein the two laser beams are produced by a dual-frequency laser producing two beams with the same axis having different frequencies and orthogonal polarizations. 
     
     
       3. The device as claimed in  claim 1  wherein the polarizer is a polarizer at 45°, combining the two laser beams in a single polarization in balanced proportions. 
     
     
       4. The device as claimed in  claim 1  wherein the polarizer is a polarizer combining the two laser beams in a single polarization in different proportions. 
     
     
       5. The device as claimed in  claim 1  wherein the output of the device is located downstream from a transmission output of the resonant cavity. 
     
     
       6. The device as claimed in  claim 1  wherein the output of the device is located upstream from the resonant cavity. 
     
     
       7. The device as claimed in  claim 1  wherein the resonant cavity comprises a secondary reflected output, and in that the polarization splitter receives a fraction of the radiation from the secondary reflected output of the resonant cavity. 
     
     
       8. The device as claimed in  claim 7 , wherein a beam splitter is provided upstream from the resonant cavity for transmitting a fraction of the radiation from the reflected output of the resonant cavity to the polarization splitter. 
     
     
       9. The device as claimed in  claim 7  wherein the servo control means include two photodiodes placed so as to each receive a respective polarization from the polarization splitter. 
     
     
       10. The device as claimed in  claim 9 , wherein the interrogation device of the cavity includes:
 a frequency generator, 
 a phase modulator placed on the path of the light beams upstream from the resonant cavity, the phase modulator being controlled by the frequency generator for modulating the phase of these beams at a determined frequency, 
 and a mixer at the output of each photodiode for modulating the output signal of the photodiode at the determined frequency before applying it to light frequency adjustment inputs of the laser beams. 
 
     
     
       11. The device as claimed in  claim 1  wherein the laser beams are produced by a dual-frequency laser including an electronic signal input for adjusting the half-sum of the light frequencies of the two beams and an input signal for adjusting the difference of the two light frequencies, or alternatively an electronic signal input for adjusting one of the light frequencies and another electronic signal input for adjusting the other light frequency. 
     
     
       12. The device as claimed in  claim 1  further comprising, on a main output of the device, downstream from the polarizer, a photodiode, a photomixer or a terahertz antenna for producing an amplitude-modulated signal at the beat frequency of the light radiation emerging from the polarizer.